1. Field of the Invention
The invention relates generally to the field of neutron well logging instruments that use neutron generators. More specifically, the invention relates to methods for controlling operating parameters of a neutron generator in such instruments to optimize neutron output and neutron generator operating lifetime.
2. Background Art
Neutron well logging instruments are known in the art for evaluation of physical characteristics of subsurface formations penetrated by wellbores. Neutron well logging instruments typically include a source of neutrons having what is termed “high” energy, for example in a arrange of 2 to 14 million electron volts (eV), and one or more radiation detectors placed at selected distances from the neutron source. The neutrons from the source enter the formations surrounding the wellbore in which the instrument is disposed. The one or more radiation detectors count radiation events, for example, inelastic gamma rays, epithermal or thermal neutrons, or capture gamma rays. Numbers of such radiation events, and/or their time and energy spectral characteristics, are related to physical parameters of interest of the surrounding formations. Such parameters include, as non-limiting examples, fractional volume of pore space (porosity), and mineral composition of the rock grains making up the various formations.
Neutron well logging instruments are known to include isotopic (chemical) sources, which include one or more radioisotopes emitting neutrons at a more or less constant rate, and electronic neutron generators. Electronic neutron generators include a pulsed neutron generator that is configured to emit controlled duration pulses or “bursts” of neutrons. Other examples of electronic neutron generators provide continuous neutron generation.
Generally, neutron generators known in the art include a hermetically sealed envelope or tube that is substantially evacuated. A reservoir (which in some examples is embodied in a filament) in the tube includes adsorbed deuterium. The reservoir is controllably heated, typically by an electrical resistance heater. An ion generator or ion source is disposed in the tube, typically near one end of an accelerating column. When actuated with electrical power, the on generator ionizes the deuterium and/or tritium released by heating the reservoir. A target is disposed at the opposite end of the tube and is charged to high voltage relative to the ion generator so as to attract and accelerate the ions produced by the ion generator. Ions strike the target, which may in some neutron generators include tritium and/or deuterium therein, so as to create helium and free neutrons by a fusion reaction. For purposes of well logging, it is important that the number of neutrons in each burst, the burst timing, and the energy of such neutrons are well controlled. Two operating parameters of a pulsed neutron generator that are controllable to affect the neutron output are the ion beam current and the voltage applied to the target (“high voltage”). One example of a pulsed neutron generator and control system therefor is described in U.S. Pat. No. 4,298,805 issued to Dennis. Ion beam current is typically controlled by adjusting the current applied to the reservoir heating element (e.g., the filament). The greater the ion beam current, the greater the neutron output. Increasing the filament power (by increasing current and/or voltage) increases the filament temperature, and thus the number of gas molecules inside the tube available for ionization and acceleration to the target.
In general, a pulsed neutron generator should be operated at the maximum, reliable high voltage applied to the target that the electrical insulation surrounding the neutron generator tube and the tube itself can sustain. At such level of high voltage, a certain ion beam current will be necessary to achieve a nominal neutron output. At higher voltages, less ion beam current will be required for the same neutron output, which usually results in less erosion of the target, thus longer target life. Typical neutron generator regulation techniques provide that with decreasing neutron output, the ion beam current is increased (e.g., by increasing filament power), and vice versa, to compensate and maintain the desired neutron output. Pulsed neutron generator tubes are subject to high voltage leakage current. Leakage currents typically result from electrons leaving the high voltage electrodes (such as the target) and moving to a focusing or other lower voltage electrode disposed inside the tube. Such leakage current is an unavoidable result of the high voltage applied across the accelerator column, and the magnitude of such leakage current may depend on the age and physical condition of the neutron generator. Leakage current can also flow between high voltage electrodes outside the neutron generator tube, through insulating media or along surfaces of insulators. The total electrical current that must be provided by the high voltage power supply will be related to the sum of the ion beam current and the leakage currents. When the ion beam current plus the leakage current exceeds the maximum current available from the high voltage power supply, the leakage current may cause the numbers of neutrons produced by the pulsed neutron generator to decrease, and such decrease in neutron output can no longer be compensated by further increasing the ion beam current. Regulation of high voltage and ion beam current are complicated by the fact that leakage current is frequently not predictable or readily measurable. Thus, during operation of a pulsed neutron well logging instrument, intermittent events of excessive leakage current having variable duration may occur.
What is needed is a technique for controlling neutron generator ion beam current and high voltage that can provide optimum neutron output even if the leakage current becomes excessive.